The invention relates to controlling the fresh air and burnt gases introduced into an internal combustion engine during transitions between the purging of a nitrogen oxides trap and the regeneration of a particulate filter.
More especially, the invention relates to engines provided with a loop for the partial recirculation of exhaust gases, also called an “EGR circuit”.
Current pollution standards require the implementation of policies that result in a reduction in the emissions of nitrogen oxides (NOx) and particulates. One means of obtaining this objective is the implementation of systems that process exhaust gases, such as a nitrogen oxides trap or a particulate filter, in addition to an EGR circuit. Nevertheless, the use of these processing systems requires maintenance phases carried out under specific conditions of combustion. Indeed, the particulate filter requires the exhaust gases to be at an increased temperature to be regenerated and, more especially, as rapid a rise in temperature as possible. The nitrogen oxides trap must be purged of nitrogen oxides and sulfur oxides (SOx) at a hot temperature and in a reducing environment and this purging also requires as rapid a rise as possible in the temperature of the exhaust gases. Moreover, for their maintenance, these processing systems periodically require the implementation of specific alternate modes of combustion.
On the other hand, so as to respect the thresholds of polluting emissions imposed by current standards, it is advisable to control the quantities of partially recirculated gases and the quantities of fresh air admitted into the engine whatever the current mode of combustion of the engine. These modes of combustion can, for example, be a normal mode of combustion, a mode of combustion specific to the regeneration of the particulate filter or a mode of combustion specific to purging the nitrogen oxides trap. It is important, therefore, to control the quantities of partially recirculated gases and the quantities of fresh air admitted into the engine during the different modes of combustion used to ensure the maintenance of the exhaust gas treatment systems such as the particulate filter and the nitrogen oxides trap. Moreover, it appears necessary to manage the transition phases from one mode of combustion to the other for total control of the flow rates of air admitted and of the partially recirculated exhaust gases, and for guaranteeing a good driving experience in the vehicle. This periodic implementation of the specific alternate maintenance phases can, in fact, impair the quality of the driving experience, the fuel consumption and the management of polluting emissions.
There are methods that exist for controlling the flow rates of air and of partially recirculated gases that utilize control structures to regulate said flow rates. For example, reference can be made to French patent application FR 2 886 339, filed in the name of the applicant, which describes a structure called “rich mixture control structure” used to regulate the flow rate of air and the flow rate of partially recirculated gases in rich mixture, that is to say when the ratio between the fuel mass and the oxidant mass used by the engine during combustion is close to 1. However, this rich mixture control structure requires the use of two flow rate regulators that have to be calibrated in terms of the different modes of combustion in progress, which takes a lot of time to develop. Reference can be made to international patent application WO 2007/063258, also filed by the applicant, which describes a structure called “lean mixture control structure” used to regulate the flow rate of air or the flow rate of partially recirculated gases in lean mixture, that is to say when the ratio between the fuel mass and the oxidant mass used by the engine during combustion is less than 1. However, this lean mixture control structure utilizes just one regulator; it is not therefore adapted for a rich mixture control structure which requires the flow rates of air and of partially recirculated gases to be regulated simultaneously.
However, the rich mixture control structure is adapted for a mode of combustion during the purging of the nitrogen oxides trap and the lean mixture control structure is sufficient for a mode of combustion during the regenerating of the particulate filter. On the other hand, these maintenance phases require the exhaust gases to be at a high temperature. So as to keep the exhaust gases at a high temperature, it is judicious to alternate these two modes of combustion. Indeed, the fact of returning to a normal mode of combustion between regenerating the particulate filter and purging a nitrogen oxides trap can lead to loss of the high temperature gain acquired for the following hot mode.
It is possible to quote documents describing interest in controlling this change-over between combustion modes during regeneration of the particulate filter and purging the nitrogen oxides trap. For example, it is possible to quote American patent application US 2003/0110760 which describes a torque management method during transitions between phases for purging a nitrogen oxides trap and regenerating a particulate filter. However, the management of the air flow rate is established by means of an air flow rate set point calculated from the engine torque and there are no details as to how to control the air flow rate during transitions between the modes of combustion. It is also possible to quote British patent application GB 2 408 003 which describes a means for controlling the phases for purging a nitrogen oxides trap and for regenerating a particulate filter triggered from the estimation of mass of sulfur oxides and particulates present in the exhaust. However, this document does not disclose any means for managing the air flow rate during phase transitions. Moreover, other documents describe means for triggering the change-over between phases for purging a nitrogen oxides trap and regenerating a particulate filter with different criteria, such as, for example, European patent application EP 1 510 671, which utilizes estimation of the mass of sulfur oxides and particulates present in the exhaust, American patent application US 2006/0070373 and international patent application WO 00/32911 which utilize the temperature within the nitrogen oxides trap, European patent applications EP 1 538 311 and EP 1 102 920 which utilize the quantities of particulates and sulfur stored in the exhaust and international patent application WO 2005/073525 which utilizes a lambda factor for excess air in the particulate filter. However, these documents do not describe how to control the air flow rate during transitions between phases for purging the nitrogen oxides trap and regenerating the particulate filter.